As-cast Films Research Articles

Nonequilibrium Polymer Rheology in Spin-Cast Films

A new experimental approach is introduced to probe the rheology of >or=100 nm-thick liquid polymer films. As-cast films were found to have a substantially reduced effective viscosity compared to annealed films. The reduced viscosity is explained in terms of nonequilibrium chain conformations giving rise to a reduced entanglement density caused by the rapid quenching of the film during spin coating. Unexpectedly long annealing times at high temperatures are required for the films to recover their bulk rheology.

Time-resolved SAXS studies of self-assembling process of palladium nanoparticles in templates of polystyrene- block-polyisoprene melt: Effects of reaction fields on the self-assembly

Nanocomposites composed of palladium (Pd) nanoparticles, (Pd) n, and a symmetric diblock copolymer (dibcp) of poly(styrene)- block-poly(isoprene) (PS- b-PI) were prepared via thermal reduction of palladium acetylacetonate, Pd(acac) 2, by isothermal heat treatments of the as-cast dibcp films containing Pd(acac) 2 by 1 wt%, at temperatures below and above the order–disorder transition temperature ( T ODT) of the dibcp. Effects of the reaction field on the thermal reduction process of Pd(acac) 2 were studied by setting the field either in the disordered state or the ordered state of the dibcp. The reduction process was studied in situ and at real time by time-resolved small-angle X-ray scattering. In the as-cast films, a larger fraction of Pd(acac) 2 is preferentially incorporated in PS lamellar microdomains, seemingly due to stronger interactions of Pd(acac) 2 with PS than with PI. The spatial concentration fluctuations of (Pd) n in the final stage of the reduction reflect the memory of the concentration fluctuations of the dibcp before the reduction, either in the disordered state or in the ordered state. We found that the scaled scattering form factor of (Pd) n is universal with time and temperature of the reduction. This reveals that the reduction mechanism of Pd(acac) 2 and the growth mechanism of (Pd) n do not depend on temperature; they are same below and above T ODT. However, the reduction rate and the growth rate as well as the concentration of (Pd) n in the final stage depend on temperature: the higher the temperature is, the larger are the rate and the concentration.

Simultaneous Block Copolymer and Magnetic Nanoparticle Assembly in Nanocomposite Films

We investigate self-assembly of nanocomposite films composed of lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and PMMA-grafted magnetite (Fe3O4) nanoparticles (NPs). The Fe3O4 NPs are grafted with PMMA brushes with molecular weights ranging from 2700 to 35 700 g/mol. For the NP with the lowest molecular weight brush, the morphology of the nanocomposite film depends on the NP concentration (ϕNP). At low ϕNP, the block copolymer self-assembles into mixed morphology of perpendicular lamellae (⊥Lam) and parallel lamellae (∥Lam), and the ⊥Lam are stabilized by individual NPs or small NP aggregates. The NPs can also retard the dynamics of self-assembly of block copolymer films. At high ϕNP, NPs form small aggregates which inhibit the formation of a lamellar structure. As the molecular weight of the PMMA brush increases to 13 300 or 35 700 g/mol, the Fe3O4 NPs form aggregates in the as-cast nanocomposite films, and this behavior is attributed to aggregation of NPs in the solution state. Since the size of NP aggregates is larger than the copolymer domain size, the block copolymer has to self-assemble around these aggregates. The magnetic properties of these nanocomposite films are characterized, and typical superparamagnetic behavior is observed.

Highly Uniaxial Orientation in Oligo(p-phenylenevinylene) Films Induced During Wet-Coating Process

Highly uniaxial alignment of pi-conjugated oligo(p-phenylenevinylene)s was spontaneously achieved in the as-cast films by simply spin-coating the solution onto mechanically rubbed polymer alignment layers such as poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS) film, resulting in a high dichroic ratio (A(//)/A(perpendicular)) of 41.0 and an order parameter ([A(//) - A(perpendicular)]/[A(//) + 2A(perpendicular)]) of 0.93 estimated from the polarized absorption spectra of the films, and its application to organic photovoltaic devices was also demonstrated to achieve a high anisotropy in photosensitivity.

Development of Meloxicam-Loaded Electrospun Polyvinyl Alcohol Mats as a Transdermal Therapeutic Agent

This study reports on the use of electrospun polyvinyl alcohol (PVA) nanofiber mats loaded with meloxicam (MX) as a transdermal drug delivery system. The amounts of MX loaded in the base PVA solution (10% w/v solution) were 2.5, 5, 10 and 20% weight, based on the dry weight of PVA (% wt). The average diameters of these fibers ranged from 121–185 nm. In all concentrations of MX loaded in spun PVA fiber mats, an amorphous nanodispersion of MX with PVA was obtained. Both the degree of swelling and the weight loss of the electrospun PVA mats were greater than those of the as-cast PVA films. The tensile strength of the as-spun fiber mats was lower than that of the as-cast PVA films, but the strain at the maximum of the as-spun fiber mats was about six times higher than that of the as-cast PVA films. The skin permeation flux of the MX permeated from MX-loaded as-spun PVA were significantly higher than from MX-loaded as-cast PVA films, and increased when the MX content in both MX-loaded as-spun PVA and MX-loaded as-cast PVA films was increased. Our research suggests a potential use for MX-loaded electrospun PVA mats as a transdermal drug delivery system.

Thin Poly(styrene-block-4-hydroxystyrene) Block Copolymer Films Spin-Coated Directly on Topographic Prepattern Substrates

We have investigated the formation of as-cast thin films of a poly(styrene-block-4-hydroxystyrene) (PS-b-PHOST) copolymer spin-coated directly on topographic prepattern substrates. Either wetting or dewetting of a polymer thin film on the elevated regions occurs in the nonequilibrium state during spin-coating with solvent vapor saturated and strongly depends on the dimensions of the prepatterns. The ratio of periodic unit area to elevated one of a prepattern (β value) is found as one of the most important factors for wettability of a thin film. The dewetting of a thin film, guided by the edges of either elevated individual periodic lines or mesas, took place with self-assembled block copolymer nanostructure when the β value was greater than a critical value of ∼4 in our system.

Highly Luminescent BODIPY-Based Organoboron Polymer Exhibiting Supramolecular Self-Assemble Structure

A novel class of rod-coil type-organoboron polymers with p-phenylene-ethynylene as the rod segment and long alkyl chain (decyl group) as coil segment has been prepared from a Sonogashira-Hagihara coupling reaction of a BODIPY-based monomer having bis-iodophenyl and decyl groups with diyne monomers, 1,4-diethynylbenzene (a), 1,4-diethynyl-2,5-bis(trifluoromethyl)benzene (b), and 2,7-diethynyl-9,9-dihexyl-9H-fluorene (c). The characterization by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the strong tendency of the obtained polymers, 2a, 2b, and 2c, to self-assemble into particles in solution and as-casted films on a micron-nm scale. Especially, 2a showed the presence of nm-sized particles and micron-sized fiber-like structures formed by aggregation of each particle. Further, in CHCl3, the gelation of 2a by three-dimensional aggregation of each fiber was observed at room temperature after 24 h. Their luminescent properties showed high energy transfer efficiency from pi-conjugated polymer linkers to BODIPY moieties (PhiF > 71%).

Efficient Poly(3-hexylthiophene)-Based Bulk Heterojunction Solar Cells Fabricated by an Annealing-Free Approach

A simple method for preparing spin-coated photoactive films with high crystalline poly(3-hexylthiophene) (P3HT) without annealing was developed. It involves the incorporation of a small amount of the P3HT monomer, 3-hexylthiophene (3HT), into the P3HT/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solution from which the films were prepared. The UV−visible spectrum of the thus-prepared film includes pronounced crystallization peaks with higher absorbance over the whole range of wavelengths, 450−650 nm, than that of the film prepared from a coating solution without 3HT. X-ray diffraction (XRD) data also reveal a smaller interlayer distance between conjugated backbones in the 3HT-processed film than the conventional film. The TEM image of the as-cast 3HT-processed film demonstrates a nanoscaled phase separation and interpenetrating network morphology like those of a thermally annealed conventional film. Polymer solar cells based on such 3HT-processed bulk heterojunction films exhibit an enhanced photocurrent and fill factor without any pre- or postannealing treatment and a similar power conversion efficiency and incident photo-to-electron conversion efficiency to those of conventional annealed solar cells, indicating the feasibility of this annealing-free approach for fabricating high-efficiency polymer solar cells.

Highly Disordered Polymer Field Effect Transistors: N-Alkyl Dithieno[3,2-b:2′,3′-d]pyrrole-Based Copolymers with Surprisingly High Charge Carrier Mobilities

A series of novel electroactive and photoactive conjugated copolymers based on N-alkyl dithieno[3,2-b:2',3'-d]pyrroles (DTP) and thiophene (TH) units (DTP-co-THs) were synthesized using a Stille coupling reaction and exhibited molecular weights of 1.6 x 10(4) to 5.0 x 10(4) g/mol. The incorporation of soluble substituted thiophenes and planar DTP units resulted in low band gap, highly conductive polymers. DTP-co-THs exhibited excellent solubility in common organic solvents and formed high-quality films. Optical characterization revealed that the band gaps of DTP-co-THs were between 1.74 and 2.00 eV, lower than regioregular poly(3-alkylthiophenes). Electrochemical characterization showed that the HOMO energy levels of DTP-co-THs are between -4.68 and -4.96 eV. When doped, DTP-co-THs exhibited high conductivities up to 230 S/cm with excellent stability. The different thiophene substituent patterns' effect on the polymers' optical and electronic properties was then examined by density functional theory computations. The microstructure and surface morphologies of poly(2,6-(4-dodecyl-4H-bisthieno[3,2-b:2',3'-d]pyrrole)-random-2,5-(3-dodecylthiophene)) (P4) and poly(2-(4,4'-didodecyl-2,2'-bithiophen-5-yl)-4-octyl-4H-bisthieno[3,2-b:2',3'-d]pyrrole) (P6) thin films were studied by X-ray diffraction and atomic force microscopy. As-cast P4 and P6 thin films exhibited poorly defined, randomly ordered lamellar structure that improved significantly after thermal annealing. Field effect transistor devices fabricated from P4 and P6 showed typical p-channel transistor behavior. Interestingly, the mobilities of as-cast, less ordered samples were much higher than those observed after annealing. The highest values of maximum and average mobilities were observed for the polymer P6 as-cast (0.21 and 0.13 cm(2) V(-1) s(-1), respectively). One of our goals was to test the idea that high mobility and excellent electrical and structural reproducibility could perhaps be achieved by the creation of amorphous pi-conjugated materials that could possess long arrange pi connectivity on the microscopic scale. The results of these studies strongly suggest that the presence of highly ordered microcrystalline structures in thin films of organic semiconductors is not necessary for excellent performance of organic transistors.

Morphology Development in Asymmetric Poly(styrene-b-tert-butylacrylate) Thin Films by Solvent Annealing

Morphology dependence of asymmetric poly(styrene-b-tert-butylacrylate) [P(S-b-tBA)] thin film on casting solvent, annealing solvent and annealing time was investigated. Two casting solvents, toluene and propylene glycol monomethyl ester acetate (PGMEA), and two annealing solvents, p-xylene and tert-butylacrylate (tBA), were studied. The casting solvent dominated the initial morphology. P(S-b-tBA) film cast from toluene resulted in inverted phase (PtBA cylinders in PS matrix) and the one cast from PGMEA led to normal phase (PS cylinders in PtBA matrix) morphology. The annealing solvents preference to either the majority or the minority block in combination with the annealing time controlled the orientation of the cylinders. When the annealing solvent preferentially interacted with the majority block in the as-cast film, a metastable highly ordered perpendicular cylinder morphology was trapped. The best conditions to obtain perpendicular PS cylinders were casting P(S-b-tBA) film from PGMEA and annealing in tBA vapor for 20 and 25 min on silicon and NaCl substrates

Lamella-to-Lamella Transition and Effect of Coil-Stretching on Crystallization in a Rod−Coil Diblock Copolymer Containing Poly(ϵ-caprolactone)

We have studied the phase properties of a diblcok copolymer poly(ϵ-caprolactone)-block-poly(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PCL-b-PMPCS) with a PMPCS volume fraction of ∼59%. The mesogen-jacketed liquid crystalline (LC) PMPCS block was amorphous in the as-cast films; and would transform into a columnar (Φ) LC phase upon the first heating and thereafter serve as rod in the copolymer studied. On the basis of simultaneous measurement of small- and wide-angle X-ray scattering (SAXS and WAXS), we identified that the original microphase-separated lamellar morphology observed in the as-cast films with amorphous PMPCS block could evolve into a new lamellar morphology with a significantly increased long period. This lamella-to-lamella transition was triggered by the amorphous-to-LC transition and was irreversible. WAXS results revealed that the chain axes of the PCL block in crystal and the rod-like PMPCS block in Φ phase were all parallel to the microphase-separated lamellar normal. Since the melting temperature of PCL crystal was well below the glass transition temperature of PMPCS block, crystallization of the PCL block was taken place in one-dimensional confined environment. Compared with those confined by the amorphous PMPCS blocks, the PCL blocks were more stretched in the confinement constructed by the Φ phase of PMPCS blocks, and thus were crystallized faster with increased fold length. This work demonstrated a lamellar phase morphology evolution based on LC phase formation of the rod block, and also indicated an effect of the degree of coil stretching on polymer crystallization.

Crystalline Structure in Thin Films of DEH−PPV Homopolymer and PPV-b-PI Rod−Coil Block Copolymers

The rod orientation and crystalline packing of a model semiconducting rodlike polymer, poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (DEH−PPV), is shown to affect the self-assembly of weakly segregated rod−coil block copolymers. The in-plane packing of DEH−PPV rods in lamellar poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene-b-isoprene) (DEH−PPV-b-PI) diblock copolymers is nearly identical to that observed in DEH−PPV homopolymers for compositions ranging from 0.42 to 0.82 vol % coil block. The crystal structure of DEH−PPV, characterized by grazing incidence X-ray diffraction and electron diffraction, consists of a tetragonal unit cell having c = 0.665 nm with a = b = 1.348 nm. The polymer chain axis is aligned along the [001] direction, and the nearest neighbor rod−rod spacing along 〈110〉 is 1.0 nm. As-cast thin films of DEH−PPV homopolymer demonstrate chain alignment primarily perpendicular to the substrate in 5100 g/mol homopolymer, while for 3500 g/mol homopolymer the chains align both perpen...

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter EmBn block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E224B114 exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.

Soluble Star-Shaped Molecules Based on Thiophene Derivatives as Organic Semiconductors for Field-Effect Transistor Applications

New star-shaped crystalline molecules have been synthesized through Horner-Emmons reactions using hexyl-substituted thiophene-based carbaldehydes as dendrons and [1,2,4,5-tetra-(diethoxy-phosphorylmethyl)-benzyl]-phosphonic acid diethyl ester as the core units; these molecules have been fully characterized. Three thiophene-based star-shaped molecules exhibit good solubility in common organic solvents and good self-film-forming properties. They are intrinsically crystalline as they exhibit well-defined X-ray diffraction patterns from uniform and preferred orientations of molecules. The semiconducting properties of the star-shaped molecules have been evaluated in organic field-effect transistors. Three crystalline conjugated molecules, 4, 8, and 12, exhibit carrier mobilities as high as 6.0 (±0.5) × 10-3, 2.5 (±0.5) × 10-4, and 2.5 (±0.5) × 10-2 cm2·V-1·s-1, respectively. The dithienothiophene dendrons in 12 induce easy crystallization and small crystallite formation even in as-cast films and can be found t...

Release characteristics of four model drugs from drug-loaded electrospun cellulose acetate fiber mats

Ultra-fine fiber mats of cellulose acetate (CA; Mw≈30 000Da; degree of acetyl substitution≈2.4) containing four different types of model drugs, i.e., naproxen (NAP), indomethacin (IND), ibuprofen (IBU), and sulindac (SUL), were successfully prepared by electrospinning from 16% w/v CA solutions in 2:1 v/v acetone/N,N-dimethylacetamide (DMAc). The amount of the drugs in the solutions was fixed at 20wt.% based on the weight of CA powder. The morphology of the drug-loaded electrospun (e-spun) CA fiber mats was smooth, with the average diameters of these fibers ranging between 263 and 297nm. No presence of the drug aggregates of any kind was observed on the surfaces of these fibers, suggesting that the drugs were encapsulated well within the fibers. After submersion in the acetate buffer solution at 37°C for 24h, the drug-loaded e-spun CA fiber mats swelled particularly well (i.e., 570–630%), while the corresponding solvent-cast film counterparts did not. The release characteristics of the model drugs from both the drug-loaded CA fiber mats and the drug-loaded as-cast CA films were carried out by the total immersion method in the acetate buffer solution at 37°C. At any given immersion time point, the release of the drugs from the drug-loaded e-spun CA fiber mats was greater than that from the corresponding as-cast films. The maximum release of the drugs from both the drug-loaded fiber mats and films could be ranked as follows: NAP>IBU>IND>SUL.

Vitamin-loaded electrospun cellulose acetate nanofiber mats as transdermal and dermal therapeutic agents of vitamin A acid and vitamin E

The present contribution reports the use of mats of electrospun cellulose acetate (CA; acetyl content = 39.8%; M w = 30,000 Da) nanofibers as carriers for delivery of the model vitamins, all-trans retinoic acid or vitamin A acid (Retin-A) and α-tocopherol or vitamin E (Vit-E). The amounts of Vit-E and Retin-A loaded in the base CA solution [17% w/v in 2:1 v/v acetone/ N, N-dimethylacetamide (DMAc)] were 5 and 0.5 wt% (based on the weight of CA), respectively. Cross-sectionally round and smooth fibers were obtained. The average diameters of these fibers ranged between 247 and 265 nm. The total immersion of the vitamin-loaded as-spun CA fiber mats in the acetate buffer solutions containing either 0.5 vol % Tween 80 or 0.5 vol % Tween 80 and 10 vol % methanol was used to arrive at the cumulative release of the vitamins from the fiber mat samples. The same was also conducted on the vitamin-loaded solution-cast CA films for comparison. In most cases, the vitamin-loaded as-spun fiber mats exhibited a gradual and monotonous increase in the cumulative release of the vitamins over the test periods (i.e., 24 h for Vit-E-loaded samples and 6 h for Retin-A-loaded ones), while the corresponding as-cast films exhibited a burst release of the vitamins.

Radiation-induced conductivity control in polyaniline blends/composites

Polyaniline (PANI) blends with chlorine-containing polymers and copolymers and composites with HCl-releasing compounds were prepared to investigate their radiation response in terms of induced conductivities. Blends of non-conductive PANI with poly(vinyl chloride) (PVC), poly(vinylidene chloride- co-vinyl acetate), [P(VDC- co-VAc)], poly(vinylidene chloride- co-vinyl chloride), [P(VDC- co-VC)] were prepared in the form of as-cast films. A number of blends which are different in composition were exposed to gamma radiation and accelerated electrons to various doses, and the effects of irradiation type and composition of polymers on the conductivity of films were investigated by using conductivity measurements and UV–vis and FT-IR spectroscopy. The results clearly showed that ionizing radiation is an effective tool to induce and control conductivity in the blends of PANI-base with chlorine-carrying polymers as well as its composites prepared from HCl-releasing compounds such as chloral hydrate. The main mechanism behind this radiation-induced conductivity is in situ doping of PANI-base with HCl released from partner polymers and low molecular weight compounds by the effect of radiation.

Surface Segregation and Formation of Silver Nanoparticles Created In situ in Poly(methyl Methacrylate) Films

Nanocomposite films have been prepared by the thermal decomposition of (1,1,1,5,5,5-hexafluoroacetylacetonato)silver(I) (AgHFA) in PMMA films containing 5, 10, and 20 wt % silver nanoparticles (NPs). When Rutherford backscattering spectrometry is used, as-cast films display a relatively uniform distribution of precursor in the midregion and a small excess of Ag near the surface, . Upon annealing at 185 °C, strong segregation of NPs to the surface and substrate is observed. The normalized by initial film thickness increases linearly with wt % Ag. For as-cast films, transmission electron microscopy (TEM) analysis shows that AgHFA forms small spherical domains (1−5 nm) prior to thermal decomposition. Upon preannealing at 107 °C, some aggregation of precursor is observed at the surface and substrate, consistent with the incompatibility of AgHFA with PMMA. Upon annealing at 185 °C, diffusion of the precursor to the surface and substrate occurs concurrently with NP formation. For 20 wt % Ag films, the NP diameters are 20−75 nm at the surface, 6 nm in the midregion, and 2−20 nm at the substrate. NP size and size distribution increase as the wt % Ag and annealing time increase. NPs near the surface are wet by a 1−5 nm thick polymer layer. Consistent with characterization studies, UV−visible spectroscopy shows a plasmon resonance attributed to Ag NPs. A model of the thermodynamic stability of submerged NPs is given and compared with experimental results. By understanding and controlling surface segregation and NP assembly, we can precisely control the surface conductivity and reflectivity in metal nanocomposite films.

Supramolecular formation of fibrous nanostructure in donor–acceptor dyad film

A novel oligothiophene and its fullerene derivative were synthesized and their film morphologies were investigated. AFM images revealed that the oligothiophene formed well-aligned fiber-like nanostructures in the film after being thermally annealed at its liquid-crystalline temperature. In the oligothiophene–fullerene dyad film, fibers were already formed in the as-cast film. Thermal annealing further enhanced the structural order and a long, partially aligned fibrous nanostructure with a width of around 10 nm and a length of as long as 200 nm was observed. Combined with the results of X-ray analysis, these features were ascribed to supramolecular self-assembly via π–π interaction of the oligothiophene groups. Photovoltaic properties of the molecules were also investigated.

Structural Analysis of Spider Silk Films

Due to their outstanding mechanical properties, spider silks have fascinated men for a long time. Silk is composed of proteins which can not only be processed into fibers, as found in nature, but also be cast to form films in vitro. Starting with a protein solution in hexafluoroisopropanol, we were able to cast films with different properties deviated from the two spider silk proteins employed. All as-cast films revealed an α-helixrich structure and were water soluble. However, the secondary structure of One-protein films (silk films cast from one spider silk protein) could be converted from an α-helical rich to a β-sheet rich secondary structure by post-cast treatment with methanol or potassium phosphate. The structural conversion was accompanied by a higher stability as seen by water-insolubility. Depending on the employed proteins, silk films were stable in protein denaturants such as urea and guanidinium hydrochloride. Strikingly, Two-protein films (silk films cast from a mixture of both spider silk proteins) showed properties derived from both proteins, indicating that the process of film casting based on silk proteins is closely related to film casting of traditional chemical polymers. Our results reveal novel possibilities to generate protein films for applications that demand stable biocompatible polymer films.